1. Field of the Invention
The present invention relates to a gas sensor, such as an oxygen sensor, an HC sensor, or an NOx sensor, for detecting a component of an exhaust gas emitted from, especially, a motorcycle.
2. Description of Related Art
Conventionally, there has been known a gas sensor composed of an outer cylinder, a metallic shell disposed inside the outer cylinder, and a sensor element disposed inside the metallic shell for detecting a component of a measurement gas. In a gas sensor having such a structure, a gap between the outer surface of the sensor element and the inner surface of the metallic shell is generally filled with a sealing material layer, as of glass.
For example, an oxygen sensor for automobile use is often mounted in an exhaust manifold or an exhaust pipe located near a suspension system and tires. In this case, a stone flipped from a tire may hit the sensor so that a mechanical shock acts on the sensor, or the sensor may be subjected to a strong thermal shock caused by splashing of water during exposure to high temperature. Further, the sensor element of the sensor has a coefficient of thermal expansion smaller than that of the sealing material layer. Therefore, in a glass sealing step, the sensor element receives a radial compressive force due to a thermal history (heating/cooling), so that stress concentration occurs in a boundary region between a portion of the sensor element covered with the sealing material layer and an uncovered portion. If a mechanical shock caused by a flipped stone or the like or a thermal shock caused by splashing of water acts on the sensor in such a state, a resultant stress acts at a boundary region (hereinafter referred to as xe2x80x9csealing boundary portionxe2x80x9d) between the portion of the sensor element covered with the sealing material layer and the uncovered portion, so that the sensor element is easily broken. Especially, an oxygen sensor used in a motorcycle is placed in an environment in which the oxygen sensor is likely to receive such shocks.
An object of the present invention is to provide a gas sensor in which stress caused by application of a mechanical or thermal shock on the sensor does not concentrate at the sealing boundary portion and which therefore has excellent durability.
To achieve the above object, a gas sensor of the present invention comprises an outer cylinder, a metallic shell, a sensor element, a sealing material layer, and two cushion layers. The metallic shell is joined to the outer cylinder. The sensor element is disposed inside the metallic shell and is adapted to detect a component of a measurement gas. The sealing material layer is mainly made of glass and is disposed between the inner surface of the metallic shell and the outer surface of the sensor element. The first cushion layer is disposed in contact with the end surface of the sealing material layer located on a front-end side with respect to the axial direction of the sensor element. The first cushion layer is formed of a mixture containing filler particles which are superior in heat resistance to glass contained in the sealing material layer, and binder particles which are superior in heat resistance to glass contained in the sealing material layer and are lower in softening temperature than the filler particles. The second cushion layer is disposed in contact with the end surface of the sealing material layer located on a rear-end side with respect to the axial direction of the sensor element. The second cushion layer is formed of a porous material containing glass whose softening temperature is slightly lower than that of the glass contained in the sealing material layer.
The above-described structure of the gas sensor of the present invention prevents local application of a strong bending stress onto the sealing boundary portion, which would otherwise occur when mechanical or thermal shock acts on the sensor element.
In general, the above-described effect of mitigating stress concentration can be achieved even when the cushion layer is disposed on only one side of the sealing material layer. However, in the case of application to a motorcycle, a gas sensor is used in an environment in which a strong mechanical or thermal shock may act on the sensor. Therefore, if the cushion layer is disposed on only one side of the sealing material layer, the sensor does not meet the shock resistance required for application to motorcycles. When the cushion layer is disposed on both sides of the sealing material layer, the sensor can sufficiently meet the shock resistance required for application to motorcycles.